Rough Idle & High HC

[4tDX]

I have a 1987 325i, m20b25. I recently did a reseal on the engine, everything except the valve seals. Also cleaned off all the deposits on cylinders heads. Valve adjustment. New belts. New engine and alternator mounts. Rebuilt fuel injectors.

Since then, it has had a rough/lumpy idle, but the RPM holds steady at ~650. At idle there is a smell of gasoline. The car fails for high HC, ~2000ppm!

Out of idle the car drives extremely smooth and doesn't feel like it is missing any power. No power loss on hills. The car pass HC out of idle, <220ppm.

When returning to idle the tack usually drops smoothly back down to 650RPM. Only occasionally will it dip below 650RPM then recover itself.

It occasionally throws a 1221 code.

I've been messing around with this thing for the past month and can't get it corrected. I've put ~400mi on the car. Here are the things I've checked since the reseal and couple other notes that may be helpful.

I believe the block is at ~160k and the head is at ~60k.
It has a 3 prong coolant temperature sensor.
Has a Malpassi adjustable FPR.

New spark plugs: NGK ZGR5A.
New O2 sensor.

• ICV (3 prong) tested
• Throttle Body Screw
• AFM continuity check
• Coolant Sensor continuity check at various temperatures
• Idle switch continuity check
• Fuel Pressure check (had to adjust FPR to bring fuel pressure into spec
• Injectors leak tested
• Smoke tested

Thanks in advance!

[E30NJ]

PCV hose running to valve cover?

[dnguyen1963]

Did you get OEM O2 sensor? Aftermarket ones can be unreliable or not fully compatible with E30.

[3vilE30]

Running either too rich or too lean,

1221 = bad o2 go get another one and try it.

If it's not that
Check:
temp sensor
Injector stuck open
Smoke test for leaks (I'm not the only one that will tell you this)

Also see if your throttle is completely closed at idle and check the tps. Sometimes the tps acts up and gives wrong data to the ecu which causes all sorts of stuff.

[4tDX]

Thanks everyone for the responses.

The one thing I've not tested is an OEM O2 sensor. This is the O2 sensor I got. I'll get my hands on an OEM one to test.
http://www.ecstuning.com/BMW-E30-325...ons/ES2532224/

The valve cover hose is brand new.

I've smoke tested for leaks multiple times... every time I move or disconnect something.

I've continuity tested the TPS... I believe it is functioning correctly. Closed circuit at idle, open at partial throttle, closed circuit at WOT?

I've tested the injectors for leaks and there were none. Is it possible that the ECU could be sending a constant OPEN signal to one or a pair?

[scoobydo]

you sure you havent got a slight misfire caused by weak mixture. If you go too lean it can cause high hc as it is so lean that it doesnt fire and raw fuel is expelled on the exhaust stroke.

Try this pdf

http://www.autoshop101.com/forms/h56.pdf

[3vilE30]

That o2 sensor shouldn't be giving you any problems, but I would get an OEM one just to test it out. Try unplugging it and see if it idle better. Drive it around with the o2 unplugged to see if you notice any difference. How are you finding out it's failing HC? Smog test?
If the o2 sensor is bad and you unplug the sensor you should have better idle. Make sure the car is warmed up tho.

The ecu might be acting up and firing a injector(s) when it's not supposed to.

Yeah TPS is fine

Closed - idle
Open - on throttle up to 2/3 throttle
Closed - WOT

[4tDX]

Thanks for the article Scooby, that is very detailed and helpful. Gives me some things to think about.

It does feel and sound like it is misfiring at idle.

Yes, I've been trying to pass smog test... I guess a little more history on all the test...

Test #1: 1600HC ppm @ idle; Passed @ 2500RPM (forget the number; <220 ppm)
Test #2: Found/Correct an Exhaust leak. No other leaks found. 2000HC ppm @ idle
Test #3: Found fuel pressure was ~4.0bar. Adjust FPR down to 3.0bar. Still at 2000HC ppm @ idle

I played around with the FPR a little more just out of curiosity. Raising the fuel pressure 3.0 - 4.0bar produced very little change in the idle. Reducing fuel pressure under 3.0 very noticeably made idle even worse. More stumbling.

I'll test the O2 sensor tonight.
See if I can borrow an ECU locally.

The only thing I can think to do next is get a dry/wet compression test, but it seems unlikely (or I'm just in denial) to find anything since the car feels like it pulls pretty hard and doesn't stumble out of idle or under stress.

[scoobydo]

Any sort of misfire will cause high HC. HC is unburnt fuel, misfire injector doesnt know that the car is going to fire so still squirts fuel, fuel doesnt ignite due to misfire, fuel gets expelled on the exhaust stroke as unburnt fuel rather than the normal CO+h2o+co2 and other trace elements.

[jlevie]

Is the smoke test being properly done? That means plugging the exhaust, replacing the AFM with a plug & nipple for smoke injection, and then pumping in oil smoke until a pressure of 2-4psi is reached. That smoke pressure must be held for at least 5 minutes.

[4tDX]

Smoke Test: I'm plugging the exhaust and plugging the intake after removing the air box. I have a little homemade device that is using mineral oil, but I am not able to monitor the pressure with its current design.Also, I probably have not been supplying/maintaining pressure for a consistent length of time, but I can add a regulator to the airline and make it more consistent and give it another go.

O2 Sensor: Unplugging the O2 sensor produced a slight audible change, but there was no change to the rough idle.
Drove it again this morning and it was maintaining idle at 750RPM (with the O2 sensor plugged in it is closser to 600-650RPM).
With the O2 sensor disconnected, is the ECU/DME just assuming a default air volume or density and making an adjustments?

[3vilE30]

O2 Sensor: Unplugging the O2 sensor produced a slight audible change, but there was no change to the rough idle.
Drove it again this morning and it was maintaining idle at 750RPM (with the O2 sensor plugged in it is closser to 600-650RPM).
With the O2 sensor disconnected, is the ECU/DME just assuming a default air volume or density and making an adjustments?

Normal idle is 740 +/- 40 Rpms

With the o2 unplugged it's running a set default open loop fuel trim
So I assume the o2 is the culprit

Oh yeah check the gap on the plugs make sure they're .7-.8

[4tDX]

Funny you just mention the idle. Bumped into someone at work familiar with the car and he mentioned moving to a new section on the AFM resistor track. So, I gave that a try and after warming up the car it now idles right around that 740rpm mark. However, still have poor idle.

After doing more research on the O2 sensor I'm not convinced it is the O2 sensor. Sounds like the O2 sensor may be trying to correct a bigger problem than it can handle and running in open loop can just "help" mask some other problems.

So, I decided to tackle some things I hadn't looked at before... Took off the distributor cap and rotor arm which were heavily worn. Should have payed closer attention when putting it together, live and learn...

Well this leads to my next series of questions.
Is this a sign of another problem or is it just age and mileage? They are both Bosch units with builds dates of 1996 (cap) and 2000 (rotor)?
Primary and secondary resistance on the ignition coil are 0.9 ohms and 5610 ohms (0.5 ohms & 5000 ohms respectively). Is this reasonable?
Anyone like a particular brand as I hear mixed things about both depending on who you talk to (Bosch or Bremi)? I'm not really concerned about the price of one over the other.

[3vilE30]

Hmmm. The CPS is known to cause some idle issues if its dirty or not working right. They'll test OK, but sometimes they're still bad.

Running open loop factors out anything mechanically wrong with the motor and I guess it factors out vacuum leaks as well.

I'm not sure what the operating resistance should be for the cap and rotor, but if they were severely worn you would have more than just a rough idle. I would try to exhaust all the cheaper methods first before you jump on new parts just to find out it wasn't that.

[rturbo 930]

Did you ever get this fixed? Just put my car back together, and it still has a rough idle. Sounds a lot like your problem, almost identical. Runs great, just idles like shit. Pretty sure it's running rich. Idles around 600ish rpm, about what yours does, which is a bit low from what I've read.

Last time I got it inspected (with a different block/head, but same idle issue) it read:

HC ppm @ idle:
Max: 220
Reading: 1110

Not as bad as yours, but still pretty bad

Current block/head is used, supposedly low miles. Did pretty much every gasket/seal I could do without taking the head off. I'm guessing/hoping it's something external to the engine.

Dunno what yours sounded like, but this is what mine is doing: